To decelerate a vehicle, the wheels of the vehicle are braked. In the case of motor vehicles, and in particular in the case of utility vehicles, the wheel brakes of the wheels each comprise a brake cylinder. The desired brake pressure in the brake cylinders can be generated pneumatically.
In a normal braking mode, the brake pressure is adjusted directly by the driver of the motor vehicle. The driver can transmit the driver's braking demand by operating a brake pedal. A service brake valve can be operated by means of the brake pedal and as a result the brake cylinder is supplied from a pressure reservoir.
As an alternative to the normal braking mode, a brake control unit undertakes the adjustment of the brake pressures in a pressure control mode, wherein the brake pressure is adjusted according to the specifications of the brake control unit on detecting corresponding braking requirements. Such braking requirements can for example be the implementation of ride stability functions such as an anti-lock function if the control unit detects the presence of a tendency of certain wheels to lock up. DE 10 2009 058 154 A1 discloses such a brake system, the brake control unit of which moreover undertakes the adjustment of the brake pressure in the pressure control mode for the implementation of external braking demands. External braking demands are independent of the driver's braking demand and are for example specified to the brake control unit by external driver assistance systems. As systems embodied separately from the brake control unit, driver assistance systems output signals according to the desired braking power to the brake control unit of the brake system, for example via a data bus.
In the known brake system, the brake control unit carries out control of the brake system to implement ride stability functions such as anti-locking interventions or even stability control and additional external braking demands. Further, simultaneous driver braking demands are implemented. The external braking demand is specified to the brake control unit as a setpoint deceleration value, i.e. as a value that represents the deceleration of the motor vehicle that is being targeted by the driver assistance function. In a situation in which both at least one external braking demand and a driver's braking demand are to be implemented in the pressure control mode, i.e. the driver is braking in addition to an external braking demand, the brake control unit adjusts the brake pressure on the respective brakes according to a resulting setpoint deceleration value of the vehicle deceleration. In the known brake system, the driver's braking demand and the external braking demand are combined additively. Alternatively, in the known brake system, in a “maximum” mode the brake control unit forms the maximum value from a setpoint deceleration value that is demanded by the brake system internally because of a driver's braking demand and an externally demanded setpoint deceleration value. In this case, an externally demanded braking demand is only adjusted if it is higher than the internal braking demand.
Carrying out the adjustment of the brake pressure in the pressure control mode by a brake control unit in the presence of a tendency to lock up of certain wheels is referred to as an “Anti-lock Braking System” (ABS). I.e. during each braking operation, only a braking force corresponding to the coefficient of friction of the road can be used. If the introduced braking force exceeds the maximum transferable braking force on one or more wheels, said wheels start to lock up, whereby the motor vehicle can be unstable. An Anti-lock Braking System permanently monitors the revolution rate of each wheel by means of measurement signals of revolution rate sensors and determines the respective wheel slip therefrom. This can be carried out for example by comparing the wheel speed that is determined from the wheel revolution rate with a (calculated) vehicle reference speed. If a tendency to lock up the wheel is detected by means of the wheel slip determined in this way, i.e. a specified slip limit is reached or exceeded, the brake controller undertakes control over the adjustment of the brake pressure. In this case, a reduction of the brake pressure is carried out in a first step, in order to then control the brake pressure of the wheel involved along the slip limit. In this case, the braking torque is raised further until a braking torque corresponding to the coefficient of friction of the road is reached. In principle, as a result the motor vehicle is approximately optimally decelerated and at the same time stability and steerability are maintained.
DE 38 29 951 A1 discloses a method for carrying out load-dependent control of the brake pressure on a utility vehicle that uses the components of an existing anti-lock system to thereby implement an automatic load-dependent brake function (ALB) that is acting far below the locking limit in the normal braking mode. With the known method, the brake pressure and thereby the braking force distribution are controlled axle-specifically below the locking limit, wherein interaxle braking force distribution is automatically controlled according to the analysis of the revolution rate signals that is provided by the revolution rate sensors in a slip region below the region in which the anti-lock function is active.
If a known brake system according to DE 10 2009 058 154 A1 implements an external braking demand in the pressure control mode and then detects a control demand for the anti-lock function, i.e. detects reaching or exceeding the slip limit on at least one vehicle wheel, the anti-lock function undertakes control over the adjustment of the brake pressure in the pressure control mode. During the implementation of the external braking demand, the full reservoir pressures are thus available at the pressure control valves on all axles. The adjustment of the brake pressure according to the external braking demand and the corresponding vehicle deceleration is made more difficult, and not least as a result the safety of the braking and the ride comfort are reduced. In particular, it always comes back to an undesirable jerk because of a sudden increase in the brake pressure within the anti-lock function.